The degradation characteristics of the aliphatic polyesters are not ideal. Polyglycolic acid and copolymers with a high glycolic acid component (e.g. polyglyconate) lose strength rapidly (typically in around 4 weeks). Most healing processes (e.g. fracture repair, bone healing etc) can take longer than this (typically 6 to 12 weeks). Therefore implants made of these polymers do not provide mechanical support over the full duration of healing. For these polymers mass loss generally occurs after about 1-1.5 years. The release of glycolic acid breakdown products has been linked to inflammatory reactions.
In contrast, polymers based on poly(L-lactide) (PLLA) retain their mechanical properties for much longer (typically 6-12 months) which means that they can provide mechanical support throughout the healing process. However, PLLA does not undergo complete mass loss for 3-5 years. This means that the device cannot be replaced by tissue until long after it has ceased to provide any function, if at all. As with PGA, the breakdown products released on degradation are acidic and can lead to an inflammatory response.
One way which has been attempted to create more optimal degradation rates is through the use of copolymers of lactic acid and glycolic acid (PLGA), or copolymers of L-lactic acid and D-lactic acid (PDLLA). The degradation times for these polymers are between those of PLLA and PGA However, these polymers are amorphous and have poorer mechanical properties than PGA and PLLA. Also, like PLLA and PGA they degrade by bulk hydrolysis so that significant mass loss and space generation for tissue ingrowth occurs long after loss of mechanical strength.
A further problem with materials such as PLLA and PGA is that they are brittle and implants made from them can be prone to breaking to the forces exerted on them during insertion. One way that this has been addressed is to copolymerise the PLLA or PGA with a rubber-like polymer such as poly(trimethylene carbonate). This improves the toughness of the polymer but such materials still suffer the same problems of degradation profile as PLLA and PGA.
Another biodegradable polymer which has been used is polycaprolactone (PCL). This polymer melts at around 60° C. so can be delivered to the body in a molten form after which it will set to form an implant in-situ. However, polycaprolactone has a very slow degradation rate so that mass loss in the body takes over 3 years.
It is therefore an objective of the present invention to provide a biodegradable polymer that has optimum strength and degradation characteristics for procedures where biodegradable polymers are implanted.